1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Need 45 MHZ xtal oscillator circuit

Discussion in 'Radio and Communications' started by GizmoWizard, Apr 30, 2016.

  1. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    I've been looking for (with no luck) an xtal oscillator circuit for a 45 MHz crystal: AT-cut, fundamental.

    I can only find third-overtone circuits for this frequency. Can someone help please?
     
  2. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,329
    Likes:
    585
    Location:
    Peterhead, Scotland
    Why do you want such a circuit?

    Most crystals intended for 45MHz are third overtone types because a fundamental mode crystal operating at that frequency would be rather small and fragile.

    JimB
     
  3. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    Hi, thanks for your response.

    I'm building an up-conversion shortwave receiver and need a 44.545 MHZ oscillator for the second mixer.

    All of the available crystals from Mouser or Digi-Key for that frequency are fundamental mode.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. Diver300

    Diver300 Well-Known Member

    Joined:
    Oct 19, 2007
    Messages:
    3,065
    Likes:
    101
    Location:
    England

    Well the big difference between a 3rd overtone circuit and a fundamental circuit is that the 3rd overtone circuit contains a high-pass filter to stop the fundamental mode happening. So if you have a 3rd overtone 44.545 MHz circuit, it will be filtered to stop the crystal oscillating at around 15 MHz, the fundamental frequency.

    As the 44.545 MHz fundamental crystals won't have any resonance lower than 44.545 MHz, it isn't necessary to prevent lower frequency oscillation, but it doesn't hurt to prevent it and the circuit should just work.

    Just remember that a 44.545 MHz crystal is around 37 μm thick, and is made out of quartz, which is about as brittle as glass. The crystal can only be supported at its edges, so you are not talking about a robust item.

    The Mouser / Digikey part is made by CTS, and has a load capacitance of 13 pF, and a maximum drive level of 300 μW. The ESR is anything up to 30 Ω. To keep below 300 μW at 30 Ω, the current must be less than around 3 mA. If the load is 13 pF, that means that the crystal impedance is around 300 Ω, and you should keep the voltage across the crystal to 0.8 V rms (2.33 V peak-peak).

    It would be better to keep the power down to 100 μW, and that calculation depends on the load capacitance being correct. If your circuit's load capacitance is different, the frequency will be slightly off, and there will be a different impedance.
     
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,049
    Likes:
    541
    Location:
    AZ 86334
    Crystal oscillators for
    I have always been under the impression that the filter required to prevent an overtone crystal from oscillating on its fundamental must be added to the oscillator circuit (is not in the crystal can). My evidence for this is that if I plug a third or fifth overtone crystal into the "crystal tester" built-in to my frequency counter, it shows the fundamental frequency, not what is stamped on the can. e.g., a 45MHz crystal would show about 15MHz when tested...

    If you look at the detailed data sheets for the ~45MHz crystals on DigiKey, you will find that the data sheet implies that they are third overtone, not fundamental.
     
  7. Diver300

    Diver300 Well-Known Member

    Joined:
    Oct 19, 2007
    Messages:
    3,065
    Likes:
    101
    Location:
    England
    I would agree with all of that, except that several very small surface mount crystals up to 50 MHz or so, and the only 44.545 MHz crystal at DigiKey, are fundamentals:-

    https://www.digikey.com/product-detail/en/cts-frequency-controls/406C35B44M54500/CTX678TR-ND/604571

    At 48 MHz, Digikey have 417 Fundamental, and 17 3rd overtone crystal in stock, although many of those may be repeated due to different packaging etc. and I haven't looked in detail, I've just applied the filters on the DigiKey website for the oscillation modes.

    It seems that in the very small packages, the upper frequency limit for fundamental crystals has been raised quite a lot in recent years.
     
  8. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    Gentlemen, thank you for the information.

    The CTS crystal that I have has a power rating of 10 μW typ, 100 μW max. So I'm aiming for 50 μW to ease up on the gain/noise of the post-amp. The ESR is 40 ohm and the shunt capacitance is 4 to 7 pF.

    So I can just take a standard Colpitts oscillator, for example, plug in the formulas for the two capacitors, and limit the voltage across the crystal to maintain a 50 μW drive level?
     
  9. Diver300

    Diver300 Well-Known Member

    Joined:
    Oct 19, 2007
    Messages:
    3,065
    Likes:
    101
    Location:
    England
    Yes.

    Please share the circuit.

    At 4 - 7 pF, the impedance will be quite high so the permitted voltage will be larger.
     
  10. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    I don't have an oscillator circuit to share just yet, as that was the impetus for this thread. If you hadn't reminded me of the crystal drive level I probably would have melted the quartz o_O

    As soon as I get something working or an idea, I'll drop it in here.
     
  11. Diver300

    Diver300 Well-Known Member

    Joined:
    Oct 19, 2007
    Messages:
    3,065
    Likes:
    101
    Location:
    England
    You don't melt quartz, you crack it.

    Quartz crystal are mechanical resonators. Their resonant frequency is almost entirely due to their physical properties, the mass and the stiffness. They are adjusted in production by adding mass, and the electronic circuit can only make tiny changes to the frequency.

    If you overdrive a crystal, it moves too much and cracks or suffers effects from the movement being too large.
     
  12. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    I was joking about melting the quartz.
    Back when I was designing amateur radio equipment for a living, whenever I blew a power transistor someone would inevitably yell "make sure you clean that silicon off the floor."
     
  13. BobW

    BobW Active Member

    Joined:
    Apr 28, 2010
    Messages:
    501
    Likes:
    52
    Location:
    Canada
    Is there any reason why you don't just buy a 45 MHz packaged crystal oscillator?
     
  14. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    Hi,

    I need a 44.545 MHz signal to drive a quad-ring mixer in a dual- conversion receiver. If I could find a crystal oscillator module at that frequency I'd snap it up.
     
  15. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    Have you considered using one of the Analog Device DDS chips, programing them is very easy. I been using the AD9833 device. They take very little space and programming is just over simple SPI interface. I assume your receiver has a microcontroller onboard. Well just a thought.

    [​IMG]

    http://www.analog.com/media/en/technical-documentation/data-sheets/AD9833.pdf

    You can get a premade module from ebay that would just plug into what ever pcb you have for cheap.
    [​IMG]
    http://www.ebay.com/itm/like/231902998882?lpid=82&chn=ps&ul_noapp=true

    The module has on onboard programmable PGA with a digital pot for setting gain of the output.
     
  16. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    Here is how I interfaced the Module with a microcontroller, just 4 wires.

    ad9833.PNG
     
  17. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,049
    Likes:
    541
    Location:
    AZ 86334
    Or use the "value-added custom field-programmable oscillator"service that DigiKey offers. For a small additional fee, DigiKey will deliver any of these oscillators pre-programmed to the nearest frequency possible to your specification.
     
  18. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    Wow! Many thanks for the time and effort everyone has taken to help me with this. I've got a lot of choices to consider.

    I'm embarrassed that I wasn't even aware of the existence of Digi-Key's preprogrammed oscillator service. That looks very promising, as does several of the other options on this thread.

    Again, thanks everyone.
     
  19. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,049
    Likes:
    541
    Location:
    AZ 86334
    Be aware that a lot of these programmable oscillators have too much phase noise to be used as a Local Oscillator, especially if you are multiplying the frequency before using the multiplied freq for LO injection...
     
  20. Mikebits

    Mikebits Well-Known Member

    Joined:
    May 24, 2008
    Messages:
    6,214
    Likes:
    174
    Location:
    San Diego, Ca
    I did not know of this service. Thanks Mike.
     
  21. GizmoWizard

    GizmoWizard Member

    Joined:
    Mar 12, 2006
    Messages:
    36
    Likes:
    0
    Location:
    Tennessee
    I will not be multiplying the output, 44.545 MHz will be the LO injection frequency. Low jitter and phase noise will be important.

    Since phase noise is not listed in the data sheets, do you have any suggestions on narrowing down the large number of available devices?
     

Share This Page